WITH THE APPEARANCE of the first RNA molecules, incipient life entered the
era of molecularly encoded information and progressively built the DNA-RNA-
protein triad that now rules the entire biosphere. Three key concepts were introduced in the wake of informational molecules: complementarity, contingency, and
modular assembly.

COMPLEMENTARITY

Biological information transfer is based on chemical complementarity, the relationship that exists between two molecular structures that fit one another closely.
Images such as lock and key, mold and statue, are often used to illustrate such a
relationship. In the chemical realm, complementarity is a more dynamic phenomenon than these images suggest. The two partners are not rigid. When they embrace,
they mold themselves to each other to some extent. Furthermore, the embrace leads
to binding. Its degree of intimacy is such that electrostatic interactions and other
short-range physical forces act strongly enough to prevent the association from
being disrupted by thermal jostling.

Base pairing, the support of the genetic language, is the most spectacular manifestation of chemical complementarity in biology. But it is only one of many. Every
facet of life depends on molecules that "recognize" each other. Self-assembly, the
phenomenon whereby complex structures are formed from a number of parts, rests
on complementarity relationships between the parts, as did the assembly of furniture in the old days, except that chemical parts even provide their own glue.

Take the immune system and its astonishing versatility and specificity. What
makes us resistant against polio or whooping cough--as a result of a previous
attack or vaccination--is the presence in our blood of special protein molecules,
antibodies, that specifically bind to some component, termed antigen, of the
pathogens. The cells that recognize a grafted heart or kidney as foreign, and reject

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